Turbine blades, because of their complex design, can suffer from vibration at frequencies which correspond to natural frequencies of the blades called modes. Each mode is associated with a different type of vibration such as along the rotational axis of the turbine, perpendicular to the rotational axis of the turbine, etc. To prevent excessive vibration of the blade about its normal position, normal design practice dictates that the blades are constructed such that those modes are located between harmonics of the operating frequency of the steam turbine. However, manufacturing tolerances, changes in blade attachment to the rotor, changes in blade geometry due to erosion and changes in the operating frequency of the turbine, among other factors, cause mode frequencies to approach harmonics of the operating frequency. Additionally, damaging nonsynchronous vibration may also occur. Typically, nonsynchronous vibration in a steam turbine may occur as a result of buffeting wherein a low steam flow and a high back pressure cause the random excitation of the turbine blades or as a result of turbine rotor torsional stresses.
While various methods of suppressing vibration are known including, for example, magnetic coupling as discussed in U.S. Pat. No. 4,722,668, fluid injection as discussed in U.S. 2013/0280050 and blade tuning as discussed in U.S. Pat. No. 4,878,810, there is nonetheless a need for alternative vibration prevention methods.